228 PRACTICAL ORGANIC C The caustic soda is dissolved in about half its weight °f water, and is added hot to the anthraquinone sulphoiiato <^>f~ sodium, previously mixed into a paste with the potassivn"1"* _ chlorate dissolved in about 50 c.c. of water. The niixture, wliioli forms a stiff paste, is transferred at once to a small metal pressure tube of steel or phosphor-bronze of the shape and dimensions shown in Fig. 85-1 The mixture fills it about two-tl^ircls full- ^ A sheet of asbestos "cardboard iis inserted between the body n.ri<:l the top of the vessel, and the meUil lop is then screwed firmly on. The pressure tutoe is heat eel Tor three hours in a paraffin- O1~ oil-bath, so that the therm oineter in- serted into tlie inner t vi lj>e, which contains a little po.riiftln, regis- ters 190—200°. The cliii-k violet coloured mass, after cooling, is scraped out niicl di^'^^ted with boiling water for an hour. Milk of lime is added until the violet calcium alizarate is all precipitated. Tliis ca.n l^e ,'iscer- tained in a small filtered sample by adding £t little mi He of lime, when no violet precipitate should be formed. The prcici pilule is filtered at the pump and washed with boiling- water until the filtrate is no longer red. The red filtrate contains a. little: mono- hydroxyanthraquinone, which may be precipitated. !>>r hydro- chloric acid. The calcium alizarate . 011 tlie filtor is sus- pended in a large quantity of hot water, and decomposed hy adding hydrochloric acid. The alizarin, which. sepn.ri.Ltos as an orange, flocculent precipitate, is filtered cold, \vashed o.l^out eight times with cold water, and finally dried and cryst^Llliisecl from alcohol or preferably cumene. Yield, 10 — i 5 grams. The thickness of the metal is i cm. FIG. 85. -+-61L/X Properties. Orange needles ; m. p. 28 c> - 290° ; sublimes completely at 140° without decomposition ; soluble In alkalis with a deep purple colour (sodium alizarate). . It is reduced to anthracene on heating with dry zinc dust. 1 The apparatus was made for xu by West's Gas Improvement Co. , 3VZ ilos Platting, Manchester.